For a lithium secondary cell (in this specification, a lithium secondary cell may sometimes be referred to as a secondary cell or a cell) having a high energy density which has been developed in recent years, it is important to secure safety, particularly to secure safety against overcharging, because of the high level of the energy density. The reason is as follows.
Namely, at the time of charging a lithium secondary cell, the lithium secondary cell will be in an overcharged state, if a more than the prescribed level of electric current or voltage is applied for some reasons, or if more than the prescribed level of capacity is charged. Consequently, it is likely that a positive electrode active material or a negative electrode active material, as a material constituting the lithium secondary cell, tends to be chemically unstable, or that internal short circuiting between electrodes takes place, which causes an excessive increase of the cell temperature and so on. In fact, if a lithium secondary cell is in an overcharged state, a gas may be generated by decomposition of the electrolytic solution, etc. By such generation of a gas, not only bursting, leakage, etc. of the cell is likely to occur, but also, if such a state continues, finally, the cell temperature may increase excessively, whereby the cell is likely to catch fire or explode.
Therefore, various studies have been made to prevent such overcharging, and the following methods {circle around (1)} to {circle around (4)} have, for example, been proposed.
{circle around (1)} A method for controlling a charging current by an electronic circuit (safety circuit) attached outside the cell, when the cell becomes a dangerous state by overcharging.
{circle around (2)} A method for shutting off a charging current by increasing the internal pressure of the cell by means of a gas generated by the decomposition of the electrolytic solution, etc. at the time of overcharging, thereby to let a safety valve mechanically operate to shut off the charging current.
{circle around (3)} A method for shutting off a charging current by closing pores of the separator by utilizing fusion of the separator due to the temperature rise of the cell.
{circle around (4)} A method for controlling a runaway reaction in the cell at the time of overcharging, by adding to the electrolytic solution an organic additive having an oxidation potential nobler than the positive electrode potential at the time of the full charge, to induce an oxidation reaction of the organic additive when the positive electrode potential rises by overcharging.
Whereas, in the field of a lithium secondary cell, as an improvement from the structural aspect of a secondary cell, a cell of a type has been developed in recent years in which a flat plate-like casing is constructed by using a lightweight sheathing member like a laminated film having a resin layer formed on each side of a gas barrier layer, and a cell element having a positive electrode and a negative electrode is sealed in the casing. In such a cell, a lightweight film is used as a sheathing member, whereby as compared with a conventional cell using a metal casing, the cell can be made light in weight and small in size, and the construction of the casing is simple, which is advantageous from the viewpoint of costs.
Further, as an improvement from the material aspect of particularly a lithium secondary cell among secondary cells, a lithium-nickel compound oxide such as lithium nickelate having a layered structure and having LiNiO2 as the basic composition, has attracted attention as a material having a high capacity to be substituted for LiCoO2 which has heretofore been used as a positive electrode active material for a lithium secondary cell.
Here, among the above-mentioned methods {circle around (1)} to {circle around (4)} to secure safety of the cell at the time of overcharging, the method for controlling by means of a safety circuit or the method for mechanically shutting off the charging current by a safety valve, has a problem such that it is required to mount an electronic circuit or a safety valve on the cell, whereby the cost for the cell tends to increase, or there may be a restriction in the design of the cell. Especially the cell of the type in which the cell element is sealed in the flat plate-like casing made of the above-mentioned laminated film, has a structure on which the above-mentioned safety circuit or safety valve can hardly be attached, since the laminated film has a variable shape.
Further, by the above-mentioned method {circle around (3)} for shutting off the charging current by melting the separator, there may be a case where the runaway reaction at the time of overcharging is so abrupt that melting of the separator may not be made in time, and no adequate safety can be secured at the time of overcharging. Further, in a case where a gel electrolyte as a non-fluid electrolyte is used as the electrolyte for a lithium secondary cell from the viewpoint of the liquid-holding property, it is likely that the gel electrolyte impregnated into pores of the separator tends to hinder closing of pores by melting of the separator, whereby shutting off of the charging current tends to be incomplete.
Further, in the above-mentioned method {circle around (4)} for controlling the runaway reaction in the cell at the time of overcharging, by adding to the electrolytic solution an overcharging preventive agent such as an organic additive, an overcharging preventive agent which is not directly involved in the usual charging or discharging of the cell, is added to the electrolytic solution, whereby there may be a case where an adverse effect may be presented to the cell performance other than the overcharging characteristics. Further, a gas is likely to be generated as a result of the oxidation reaction of the overcharging preventive agent at the time of overcharging, whereby corrosion of instruments by such a generated gas, or leakage, etc. of a toxic gas such as an organic gas may be feared.
As mentioned above, a sufficiently satisfactory technique as a safety measure at the time of overcharging has not yet been presented. On the other hand, with a lithium secondary cell using a lithium-nickel compound oxide expected to be a high capacity positive electrode active material for a lithium secondary cell, the reactivity of the surface of the lithium-nickel compound oxide present in a particle state in the positive electrode, is high, whereby the safety at the time of overcharging or the like tends to be relatively low.
Accordingly, for a lithium secondary cell using a lithium-nickel compound oxide as a positive electrode active material, it is desired to develop a more secure safety measure against overcharging. Further, in recent years, a higher safety has become required for a cell, as a lithium secondary cell has been used as a power source for an instrument carried by human being, such as a portable phone. Further, it is desired to reduce the cost by simplifying the safety valve or the safety circuit, and it is strongly desired to improve the essential safety of the lithium secondary cell itself.
The present invention has been made in view of the above-described prior art, and its object is to provide a lithium secondary cell which is a lithium secondary cell using a lithium-nickel compound oxide as a positive electrode active material, whereby the safety against overcharging has been more improved.